A police officer is delegated to infiltrate a soccer hooligans. He is very skeptic, but soon becomes one of them and begins to protect them. There is a conflict between loyalty to his new friends and the duty. He became fanatic and it's hard to tell if he's doing this to get trusted or just for pleasure.

I remember fans were meeting in some pub and the main character had an affair with the female bartender.

I don't remember how this exactly ends, but I think it was that the fans group is destroyed by the police. The main character is awarded for being very secret and a good actor, his bosses think he was just playing to be real, but he was real. At the very ending it is shown that he is now infiltrating new-fasist organization (doing Nazi salute) and it is left for the audience to guess if he's doing his new task or he did become a fanatic nazi.

I was sure one of the minor characters was played by Pete Postlethwaite, but nothing in his filmography matches.